import React, { useEffect, useState } from 'react'



export default function App() {
let [opacity,setOpacity]=useState(1);

  //初始化的时候添加一个定时器
  useEffect(()=>{
    const opacityTimer = setInterval(()=>{
      opacity -= 0.1;
      if(opacity<=0){
        opacity=1;
      }
      setOpacity(opacity)
    },100)

    return ()=>{
      //清理
      clearInterval(opacityTimer)
    }
  },[])


  return (
    <div>
      <h1 style={{opacity}}>ReactHook真好用</h1>
    </div>
  )
}
